The charge and discharge principle of rechargeable electrochemical devices is based on a complex reaction system making use of the migration of lithium ions between the cathode and anode and a redox reaction of lithium at each electrode as well as charge and discharge of an electric double layer formed by active carbon of the cathode. According to this system, the output voltage is as high as 3 V and also the charge/discharge cycle life is long. As a contrivance resembling this type of device, there is known a structure in which carbon is used for the cathode, lithium for the anode and a non-aqueous solution as the electrolyte as disclosed in U.S. Pat. No. 3,700,975 specification. This makes use of an electrochemical reaction of lithium at the anode and a space charge layer (electric double layer) at the cathode. The space charge layer formed at the cathode originates from anions which are a solute of the electrolyte.
From close examination of the characteristics of this device, it was found that in case, for example, anode of lithium and cathode of active carbon are simply combined by using said both materials in a same volume, the charge/discharge cycle life is very long when the drop of terminal voltage from its maximum 3.0 V does not exceed 2.0 V, but when the device is discharged till the terminal voltage drops close to 0 V, no full recovery of voltage can be attained even when the device is charged and its electric capacity is markedly reduced.
Analyzing this phenomenon, it can be inferred that such phenomenon is attributable to the facts that when the terminal voltage is in the region below about 2 V, lithium of the anode migrates excessively to the cathode, far in excess of the capacity of the electric double layer of the cathode, and lithium is discouraged from returning to the cathode even when charging is done, and that due to the accumulation of lithium on active carbon, a substantial portion of the electric double layer on the surface of active carbon is lost.
On the other hand, comparing the electric capacities of the cathode and the anode, it is seen that in case the cathode is made of lithium alone, its energy density is as high as 3,840 mAh/g, or about 204 mAh/cc. In the case of the cathode using active carbon, the quantity of electricity which it can store is far less than that storable with lithium. For instance, in case discharge is made from 3.0 V to 2.0 V in single electrode potential versus lithium, the electric capacity of the cathode using active carbon becomes 1/100 or less as compared with that of the cathode using the same volume of lithium, although the value varies depending on the kind of active carbon used.
Therefore, in case the cathode and the anode are simply combined by using the same volume of materials as in the above example, the electric capacity of the anode may become more than 100 times that of the cathode, resulting in an extreme unbalance of electrode potential, so that when the device is overcharged, it becomes difficult to restore the original capacity even if the device is later charged for the above-said reason.
As for the causes of deterioration of the device when it is overcharged, beside said unbalance of both electrodes, the fact is noted that great influence is also given by the type of alloy used for the cathode. For instance, in case the cathode is made of lithium alone there is produced dendrite of lithium by repetition of charging and discharging, and this dendrite causes shortcircuiting of both electrodes and deprives the lithium of its activity.
As means for preventing this, use of an alloy capable of both occluding and releasing lithium can suppress generation of dendrite. Thus, it is expected that the device can be made more solid against overcharging by use of such alloy. However, the rate and amount of lithium occluded and released as well as the plate strength vary according to the type of alloy used. For example, when a 50/50 (by weight) Pb/Cd alloy is used, although the plate strength increases, the lithium occlusion and release rate becomes low. Therefore, when charging and discharging are repeated at a large current value, lithium becomes unable to be uniformly dispersed in the alloy, and as a result, dendrite tends to be produced to invite a reduction of performance of the device.
The present invention has for its object to realize an improvement on balance of cathode and anode and the type of alloy used for the anode as well as the optimization of cell structure and to provide a rechargeable electrochemical device which will not deteriorate in its electrochemical properties even when discharged to 0 V and has a long charge/discharge service life and high reliability.